1. Field of the Invention
The present invention relates to an X-ray mask and a method of manufacturing the same, and more specifically, it relates to an X-ray mask including a transfer pattern having high accuracy and a method of manufacturing the same.
2. Description of the Prior Art
A semiconductor device is now being refined and implemented with higher density of integration. In a photolithographic step included in steps of fabricating a semiconductor device, therefore, formation of a finer pattern is required. Study is recently made on employment of X-rays having shorter wavelengths than ultraviolet rays, generally employed in the photolithographic step, for exposure. FIG. 29 is a sectional view showing a conventional X-ray mask employed in such a photolithographic step with X-rays.
Referring to FIG. 29, the conventional X-ray mask includes a silicon wafer 101, a membrane 102, an X-ray absorber film 104 and a support ring 109. The membrane 102 is formed on the silicon wafer 101. A window part 103 partially exposing the membrane 102 is formed in the silicon wafer 101. A transfer pattern 111 is formed on a region of the X-ray absorber film 104 located on the window part 103. The support ring 109 is set under the silicon wafer 101. The surface of the X-ray absorber film 104 can be divided into a transfer pattern region 106 provided with the transfer pattern 111, a transfer pattern peripheral region 107 located on the window part 103 to enclose the transfer pattern 111 and a window part peripheral region 108 located on the silicon wafer 101.
In a method of manufacturing this X-ray mask, the membrane 102 is first formed on the silicon wafer 101. The X-ray absorber film 104 is formed on the membrane 102. The window part 103 is formed in the silicon wafer 101. The support ring 109 is set on the lower surface of the silicon wafer 101. A resist film (not shown) is applied onto the X-ray absorber film 104. A resist pattern (not shown) for forming the transfer pattern 111 is drawn on the resist film with an electron beam. Then, development is performed for forming the resist pattern on the resist film. The X-ray absorber film 104 is partially removed by etching through the resist pattern serving as a mask, for forming the transfer pattern 111. Thereafter the resist film is removed. The conventional X-ray mask is thus prepared.
The transfer pattern 111 of the X-ray mask, directly influencing the accuracy of circuits of the semiconductor device or the like, must have high dimensional accuracy and positional accuracy. Therefore, the resist pattern drawn on the resist film formed on the X-ray absorber film 104 with the electron beam must also have high dimensional accuracy and positional accuracy.
When drawing the resist pattern on the resist film with the electron beam in the step of manufacturing the X-ray mask, the X-ray mask is fixed to a cassette (EB cassette). This EB cassette is fixed to a stage provided in a chamber of an electron beam drawing unit.
The resist pattern to be drawn is divided into a plurality of sections (fields), each of which is drawn with the electron beam. The stage carrying the X-ray mask thereon is mechanically moved between the fields for drawing the pattern. When the stage is moved, the temperature of the X-ray mask is increased due to frictional heat at contact parts between the stage and a base or heat generation from a motor employed for moving the stage. The membrane, the X-ray absorber film and the like forming the X-ray mask are thermally expanded due to the increased temperature of the X-ray mask. Consequently, the region of the X-ray mask to be provided with the resist pattern is distorted.
The temperature of the X-ray mask is also increased by the electron beam, which is an energy beam, applied thereto for drawing the resist pattern. Consequently, the region of the X-ray mask to be provided with the resist pattern is further distorted.
When starting to draw the resist pattern on the X-ray mask with the electron beam, therefore, the distortion of the region to be provided with the resist pattern is increased. FIG. 30 is a graph showing the qualitative relation between positional distortion of a conventional X-ray mask caused by temperature rise when drawing a resist pattern thereon with an electron beam in a step of manufacturing the X-ray mask and time. Referring to FIG. 30, the axis of abscissas shows the time and the axis of ordinates shows the distortion of the X-ray mask. The operation of drawing the resist pattern with the electron beam is started at a time t.sub.1. The distortion is gradually increased up to a time t.sub.2, as shown by a curve AB.
At the time t.sub.2, the quantity of heat inputted the X-ray mask balances with that of heat diffused therefrom to stabilize the temperature of the X-ray mask. Therefore, the distortion remains substantially unchanged. The distortion is thereafter substantially stabilized and the X-ray mask is substantially in thermal equilibrium up to a time t.sub.3 for completing the drawing, as shown on a line BC.
Thus, the distortion of the X-ray mask changes with time when drawing the resist pattern with the electron beam, to result in distortion of the drawn resist pattern. The rate of change of the distortion has pattern dependency, to cause dispersion in accuracy of the resist pattern. Therefore, the transfer pattern formed with the resist pattern is disadvantageously deteriorated in accuracy.
In order to prevent the transfer pattern from deterioration of accuracy, the following techniques have been proposed in general:
Japanese Patent Laying-Open No. 6-163381 (1994) discloses a technique of providing at least two marks on an EB cassette and drawing a resist pattern with an electron beam while correcting distortion of an X-ray mask resulting from temperature rise by detecting the marks. In this case, the position for drawing the resist pattern can be corrected following temporal change of the distortion. However, the position for drawing the resist pattern must be minutely re-calculated in response to the result of measurement of the distortion, disadvantageously leading to complicated control. In order to apply this technique to a conventional electron beam drawing unit, further, the electron beam drawing unit must be altered. Therefore, the cost for manufacturing the X-ray mask is disadvantageously increased due to necessary capital investment.
Japanese Patent Laying-Open No. 8-97130 (1996) discloses a technique of mounting a heater on an EB cassette for controlling the temperature of an X-ray mask. In this technique merely indirectly controlling the temperature of the X-ray mask through the EB cassette, however, it is difficult to improve the accuracy for controlling the temperature of the X-ray mask. Consequently, it is difficult to correctly control distortion of the X-ray mask caused by heat and hence the accuracy of the resist pattern cannot be readily improved.
Japanese Patent Laying-Open No. 6-36997 (1994) discloses the following technique: Temperature change of a driving table (stage) is previously monitored while drawing a resist pattern with an electron beam for obtaining data thereof. Distortion of the driving table caused by thermal expansion corresponding to the time for drawing the pattern is calculated on the basis of the data on the temperature change, the thermal expansion coefficient of the driving table and the like. The distortion is fed back to a table coordinate system or a deflection control system for the electron beam for drawing the pattern, thereby improving the accuracy of the resist pattern. In this technique of previously obtaining the data on the temperature change of the driving table with respect to the drawing time and calculating the distortion on the basis of the data, however, it is difficult to cope with dispersion in preparation of the X-ray mask when actually drawing the resist pattern. Consequently, the accuracy of the resist pattern is disadvantageously deteriorated.
Japanese Patent Laying-Open No. 63-110634 (1988) discloses an X-ray mask forming a stress relaxation pattern on an X-ray absorber film located around a transfer pattern for preventing the transfer pattern from deformation caused by stress on the X-ray absorber film. In the technique disclosed in this gazette, however, it is difficult to solve the problem of deterioration in accuracy of the transfer pattern resulting from heat applied for drawing the pattern. Further, the time for manufacturing the X-ray mask is increased due to the stress relaxation pattern formed on the overall surface of the X-ray absorber film located around the transfer pattern, to disadvantageously increase the preparation cost.
Japanese Patent Laying-Open No. 8-203817 (1996) discloses the following technique: A resist pattern is previously formed on an X-ray mask and displacement of the resist pattern resulting from stress on an X-ray absorber film is calculated. A drawing pattern for the resist pattern is rearranged to compensate for the displacement. Then, the X-ray mask is prepared with the rearranged drawing pattern. In the technique disclosed in the above gazette, however, it is difficult to solve the problem resulting from drawing of the drawing pattern in a non-stationary state with fluctuating distortion of the X-ray mask.